There is a commercial need in the semiconductor industry for metrology equipment that can provide high resolution, nondestructive evaluation of product wafers as they pass through various fabrication stages. A number of systems have been developed for the nondestructive evaluation of semiconductor samples in recent years. One such product is a Modulated Optical Reflectance (MOR) based system. This device incorporates technology described in the following U.S. Pat. Nos. 4,634,290; 4,646,088; 5,854,710; 5,074,669 and 5,978,074. For a better understanding of the MOR technology, each of these patents is incorporated herein by reference.
As described in the above-referenced patents, a basic MOR device includes an intensity modulated pump laser beam which is focused on the surface of a sample for periodically exciting the sample. For semiconductor wafers, thermal and plasma waves are generated in the sample that spread out from the pump beam spot. These waves reflect and scatter off various features and interact with various regions within the sample in a way that alters the flow of heat and/or plasma from the pump beam spot.
The presence of the thermal and plasma waves has a direct effect on the reflectivity at the surface of the sample. Features and regions below the sample surface that alter the passage of the thermal and plasma waves will therefore alter the optical reflective patterns at the surface of the sample. By monitoring the changes in reflectivity of the sample at the surface, information about characteristics below the surface can be investigated.
A basic MOR system typically includes a second laser for generating a probe beam of radiation. This probe beam is focused collinearly with the pump beam and reflects off the sample. A photodetector is provided for monitoring the power of the reflected probe beam. The photodetector generates an output signal that is proportional to the reflected power of the probe beam and is therefore indicative of the varying optical reflectivity of the sample surface.
The output signal from the photodetector is filtered to isolate the changes that are synchronous with the pump beam modulation frequency. The basic MOR system also includes a lock-in amplifier to monitor the magnitude and phase of the periodic reflectivity signal. This output signal is conventionally referred to as the modulated optical reflectivity of the sample.
In practice, the response of the sample to the pump beam is dependent on the wavelength of the laser. Further, the sensitivity of the system is also dependent on either pump or probe beam wavelength and the relationship between the pump and probe beam wavelengths. The combination of wavelengths selected by the assignee in its commercial embodiment is intended to achieve a balance between the plasma and thermal components of the total MOR signal allowing measurements over a relatively broad range of samples.
In the most common commercial application of MOR, the surface density or dosage levels of implants in silicon are measured. While the pump and probe beam wavelengths in certain existing commercial MOR systems provide good sensitivity across a relatively wide range of doses, certain regions are less sensitive than others due to the limitations inherent in the monochromatic output of lasers. Prior art systems for ion implant metrology used primarily the MOR technology. Commercial systems for ion implant metrology are marketed under the name of Therma-Probe® by KLA-Tencor Corporation of San Jose, Calif.
Early efforts to increase the sensitivity of MOR systems sought to combine MOR with other technologies in a single system. Several combinations of MOR with other technologies, e.g., MOR and Photothermal Radiometry (PTR), MOR and Spectroscopic Ellipsometry (SE), MOR and Spectrometry (a.k.a. Broadband), MOR and Junction Photovoltage (JPV), MOR and Beam Profile Reflectometry (BPR) and others, have been proposed in the prior art to enhance measurement capabilities of the ion implant metrology system. Such combinations are disclosed in the following US patents and patent applications assigned to the assignee of the present invention: U.S. Pat. Nos. 6,671,047; 6,693,401; 6,917,039; 7,060,980; and 6,678,347 and US Patent Application Publication 20070188761. MOR-like systems, which employed a heterodyne approach, as seen in U.S. Pat. Nos. 5,206,710; 5,408,327 and 6,081,127, were also considered in an effort to improve the sensitivity and performance of MOR systems.
It would be a benefit if the user was permitted to select a particular set of wavelengths in MOR system to perform certain measurements. One prior art effort to expand the wavelength measurement capability of a modulated reflectance measurement system is shown in U.S. Pat. No. 7,106,446, which is incorporated by reference in this application. One implementation of the prior art measurement system includes three monochromatic diode-based or diode-pumped semiconductor lasers. Each laser can operate as a probe beam source or as a pump beam source. The laser outputs are redirected using a series of mirrors and beam splitters to reach an objective lens. The objective lens focuses the laser outputs on a sample. Reflected energy returns through the objective lens and is redirected by a beam splitter to a detector. A filter prepares the outputs of the detector for analysis by a processor. Typically, the filter includes a lock-in amplifier that converts the output of the detector to produce quadrature (Q) and in-phase (I) signals for analysis.
The use of three different lasers provides six possible combinations where a single probe beam is used with a single pump beam. Alternately, two lasers can be used to produce different probe beams while the third laser produces the pump beam. In another variation, two lasers can produce pump beams (at different modulation frequencies) while the third produces a probe beam. Another configuration uses all three lasers to produce intensity modulated pump beams. The light reflected by the sample originating from the first laser can be monitored at the difference between the modulation frequencies of the second and third lasers. The reflected light of the second and third lasers is monitored in an analogous fashion. In this way, the prior art system provides a dynamically reconfigurable measurement system that can be optimized to measure a range of different sample types.
For another implementation, the measurement system includes a pump laser and a probe laser. One or both of these lasers are wavelength tunable. The pump laser and probe lasers are controlled by a modulator. An optical modulator is a device which can be used for manipulating a property of light—often of an optical beam, e.g. a laser beam. The type of modulator used is dependent on which property of light is to be controlled, e.g., intensity modulators, phase modulators, polarization modulators, spatial light modulators, etc.
By selectively controlling the wavelengths produced by the pump laser and/or probe laser, the operation of modulated reflectance measurement system can be optimized to measure a range of different sample types.
In some implementations of prior art measurement systems, the pump and probe lasers may be added as modular subsystems. Use of separate low-dose, mid-dose, high-dose, and all-dose modules may expand the range of the system. Each of these modules may include a pump laser and a probe laser having wavelengths that are selected to optimally analyze a particular range of implantation dosages. The all-dose module is intended to provide a wideband tool that operates over a range of dosage levels. The low-dose module, mid-dose module, and high-dose module provide insight into discrete portions of that range. The modules share a set of common components, which typically include optics, a detector and a processor. By selectively enabling or disabling the modules (alone or in combination), the operation of the operation of modulated reflectance measurement system can be optimized to measure a range of different sample types.
Photo-reflectance (PR) technology, while known to be sensitive to structural properties of semiconductor samples, particularly to strain in silicon wafers, was not previously considered to be a viable complement to the MOR technology in a combined system. In MOR, the pump beam is typically modulated at a frequency in the megahertz range. In PR, by contrast, the pump beam is modulated at much lower frequencies, typically in a few Hz to a few kHz range. Moreover, the PR system employs a polychromatic light source to obtain specific strain measurements from the designated sample over a range of wavelengths provided by the source. Measurements associated with an MOR system are specifically limited by the wavelengths of the lasers chosen for use in the system.
It is within this context that embodiments of the present invention arise.